There's a difference between overbooking a flight and bumping passengers off a flight.

As the Associated Press notes, airlines are allowed to oversell flights, and they frequently do, because they assume that some passengers won’t show up. U.S. airlines bumped 40,000 passengers last year, not counting those who volunteered to give up their seats.

According to the government, 434,000 passengers voluntarily gave up seats on the country’s largest 12 airlines last year, including nearly 63,000 on United Airlines. However, the champion of overbooking was Delta Air Lines — about 130,000 passengers on Delta agreed to give up their seats last year.

Bumping a passenger is a different matter. When it comes to forcing passengers off a flight, Southwest is the undisputed leader among the larger airlines — it bumped nearly 15,000 passengers last year, according to government figures.

Some savvy travelers will give up their seats if the airline makes a sweet enough offer. Some check their flight’s seating chart ahead of time to see if it’s sold out. If you aim to be bumped, sit near the gate agent’s desk so you can pounce before other passengers take that offer of travel vouchers, gift cards, and sometimes cash. If offered a spot on a later flight, make sure it’s a confirmed seat. And don’t check a bag.